Field of the Invention
The present invention relates to a catalyzed particulate filter, and more particularly, to a catalyzed particulate filter that minimizes an increase in back pressure and provides a longer contact time (larger contact area) between a catalyst and a fluid.
Description of Related Art
Exhaust gases from internal combustion engines such as diesel engines or a variety of combustion equipment contain particulate matter (PM). Such PMs can cause environmental pollution when emitted into the atmosphere. For this reason, gas exhaust systems are equipped with a particulate filter for capturing PM.
The particulate filter may be categorized as a flow-through particulate filter or a wall-flow particulate filter depending on the flow of fluid.
In the flow-through particulate filter, a fluid let into a channel flows only within this channel without moving to another channel. This helps minimize an increase in back pressure, but necessitates a means for capturing particulate matter in the fluid and may result in low filter performance.
In the wall-flow particulate filter, a fluid let into a channel moves to a neighboring channel and is then released from the particulate filter through the neighboring channel. That is, a fluid let into an inlet channel moves to an outlet channel through a porous wall and is then released from the particulate filter through the outlet channel. When a fluid passes through the porous wall, particulate matter in the fluid is captured without passing through the porous wall. The wall-flow particulate filter is effective at removing particulate matter, although it may increase the back pressure to some extent. Hence, wall-flow particulate filters are primarily used.
The vehicle is equipped with at least one catalytic converter, along with a particulate filter. The catalytic converter is designed to remove carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx).
The catalytic converter may be physically separated from the particulate filter, or combined with the particulate filter by coating the particulate filter with a catalyst. The particulate filter coated with a catalyst may be called a catalyzed particulate filter (CPF).
In the CPF, the catalyst is coated on the porous wall that separates the inlet channel and the outlet channel from each other, and the fluid passes through the porous wall and comes into contact with the catalyst coating. There is a pressure difference between the inlet channel and outlet channel separated by the porous wall. This allows the fluid to pass fast through the porous wall. Accordingly, the contact time between the catalyst and the fluid is short, which makes it hard for a catalytic reaction to occur efficiently.
Also, a thick catalyst coating on the porous wall allows the catalyst to block the micropores on the wall, and this may disturb the flow of the fluid from the inlet channel to the outlet channel. Accordingly, the back pressure increases. To minimize the increase in back pressure, a catalyst is thinly coated on the walls in the CPF. Thus, the amount of catalyst coating on the CPF may be insufficient for the catalytic reaction to occur efficiently.
To overcome this problem, the surface area of walls to be coated with the catalyst may be increased by increasing the number (density) of inlet channels and outlet channels (hereinafter, collectively referred to as ‘cells’). However, the increase in cell density in the limited space reduces the wall thickness. The reduction in wall thickness may deteriorate the filter performance. Therefore, the cell density should not be increased to more than the density limit.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.